As conventional inorganic antimicrobial agents, those in which an antimicrobial metal such as silver or copper is supported on apatite, zeolite, glass, zirconium phosphate, silica gel, etc. are known. Compared with organic antimicrobial agents, they have high safety, and do not evaporate or decompose, thus having a long-lasting antimicrobial effect and, moreover, they have excellent heat resistance. Because of this, antimicrobial resin compositions obtained by mixing these antimicrobial agents with various types of polymer compounds are molded into antimicrobial products in the form of various types of moldings, fibers, and films and are used in various types of applications.
Among these, vitreous antimicrobial agents comprising an antimicrobial metal such as silver, copper, or zinc have the characteristics that the glass grain size, the refractive index, and leaching properties of antimicrobial metal can easily be controlled according to the intended application, and they are therefore added to antimicrobial resin compositions for various types of applications.
For example, a silver-containing vitreous antimicrobial agent has been proposed (ref., e.g. JP-A-03-124810). Furthermore, a vitreous antimicrobial agent comprising a high concentration of zinc has been proposed (ref., e.g. JP-A-2001-26438).
Although the conventional silver-containing vitreous antimicrobial agent (also called a silver-based vitreous antimicrobial agent) has the advantage that a high antimicrobial effect can be obtained with a relatively low concentration of silver, there are the problems that, due to heat when kneading with a resin or due to exposure to UV rays after processing of the resin, degradation and deterioration of the resin itself are accelerated or the resin product is discolored, and the excellent characteristics of the resin product itself are thus often impaired. Furthermore, when the vitreous antimicrobial agent comprising silver alone as the antimicrobial component is kneaded with certain resins, such as an ABS resin or an acrylic resin, it is difficult for the antimicrobial effect to be exhibited.
On the other hand, when the vitreous antimicrobial agent comprising a high concentration of zinc alone is kneaded with a resin, it causes little degradation, deterioration, or discoloration of the resin, but compared with the silver-containing glass, the antimicrobial properties can be poor in some cases, and if an attempt is made to make the resin composition exhibit an adequate antimicrobial effect, it is necessary to increase the amount added to the resin, and as a result there is the problem that the physical properties of the resin itself are degraded.
In order to solve these problems, a vitreous antimicrobial agent comprising both silver and zinc has been proposed.
As an example of a composition comprising phosphoric acid, a vitreous antimicrobial agent comprising 0.2 to 5 wt % of Ag2O, 1 to 50 wt % of ZnO, 30 to 80 wt % of P2O5, 1 to 20 wt % of CaO, and 0.1 to 5 wt % of CeO2 has been proposed (ref. e.g. JP-A-2000-191339). A vitreous antimicrobial agent comprising 0.1 to 5 wt % of Ag2O, 1 to 50 mol % of MgO+CaO+BaO+ZnO, 30 to 60 mol % of P2O5, 2 to 40 mol % of B2O3, at most 15 mol % of Al2O3, and at most 15 mol % of an alkali metal oxide has also been proposed (ref., e.g. JP-A-2001-247726). A vitreous antimicrobial agent comprising 0.03 to 5 mol % of Ag2O, 0 to 30 mol % of ZnO+BaO, 0 to 20 mol % of B2O3, 0 to 2.5 wt % of TiO2+CeO, 0 to 5 mol % of SiO2, 20 to 55 mol % of MgO+CaO, 5 to 25 mol % of Na2O, 40 to 55 mol % of P2O5, and 0 to 5 mol % of PbO has also been proposed (ref., e.g. JP-A-8-48539).
As an example of a composition comprising no phosphoric acid, a soluble vitreous antimicrobial agent comprising 20 to 50 wt % of B2O3, 50 to 80 wt % of ZnO, at most 10 wt % of an alkaline earth metal oxide, and at most 2 wt % of Ag2O has been proposed (ref., e.g. JP-A-2000-281380). A vitreous antimicrobial agent comprising 0.1 to 5 wt % of Ag2O, 35 to 60 wt % of ZnO+MgO+CaO+BaO, 10 to 50 wt % of SiO2, 0 to 20 wt % of Al2O3, 0 to 20 wt % of Na2O+K2O+Li2O, 10 to 54 wt % of B2O3, 0 to 10 wt % of TiO2, and 0 to 10 wt % of La2O3 has also been proposed (ref., e.g. JP-A-2000-302478). A vitreous antimicrobial agent comprising 0.05 to 5 wt % of Ag2O, 0 to 30 wt % of ZnO, 0 to 20 wt % of MgO+CaO+BaO, 10 to 60 wt % of SiO2, 0 to 20 wt % of Al2O3, 0 to 4.9 wt % of Na2O+K2O+Li2O, and 10 to 60 wt % of B2O3 has also been proposed (ref., e.g. JP-A-2000-203876).
However, it is known that the vitreous antimicrobial agent comprising silver and, as a main component, P2O5, has poor hot water resistance. The glass comprising B2O3 as a main component has high hardness, it therefore abrades a metal surface of a mixer or a resin molder used for kneading it with a resin, and there is thus the problem that a metal powder formed by scraping contaminates the resin composition, thereby darkening the color of a final resin product. When a large amount of B2O3 is contained in the glass, as is the case for that comprising P2O5 as a main component, there might be problems caused by poor hot water resistance.
In order for a high antimicrobial effect, which can be provided for various types of processed resin products, to be exhibited, a glass composition having as high a concentration of zinc as possible and an appropriate amount of silver is effective. However, unless the concentrations of P2O5 and B2O3, which are glass framework-forming components, are reduced as much as possible, the relative ZnO concentration in the glass composition cannot be increased, and the problems with regard to hot water resistance, discoloration, and hardness when added to a processed resin product cannot be eliminated. Furthermore, a composition comprising a high concentration of zinc, as well as silver, which is easily reducible, can easily be prepared in a small amount such as at laboratory scale, but when commercial production of on the order of at least a few hundred kg is carried out, there is the problem that the glass is colored. It is therefore not easy to mass-produce an antimicrobial agent comprising a glass comprising silver, which has a high antimicrobial effect, and a high concentration of zinc.